The effect of a native biological control agent for Eurasian watermilfoil on six North American watermilfoils

There is increasing concern that introduced, classical biological control agents can have significant negative effects on non-target species. One alternative to classical biological control is the use of native species to control exotic pests. A North American weevil, Eultrychiopsis lecontei, is being used as a biological control agent for the introduced aquatic plant Eurasian watermilfoil (Myriophyllum spicatum) in the United States. Previously, we determined that E. lecontei did not have a significant effect on several unrelated species of native North American aquatic plants. Here, we examine the effects of this weevil on six native North American watermilfoils. In six separate experiments, individual shoots of each native watermilfoil species were exposed to zero, two or four weevil adults. Changes in length and final dry mass were determined for each shoot at the end of the experiments. We also recorded the number of weevil eggs and larvae on these native watermilfoil species at the end of the experiment. In treatments with two weevils per plant there were no significant impacts of weevils on the native watermilfoils. However, in treatments with four weevils per plant, final length of M. verticillatum after 11 days was 13% shorter than controls, and with four weevils final dry mass of M. alterniflorum was 65% less than controls and M. humile 43% less. Weevils laid fewer eggs on all native watermilfoil species than on M. spicatum controls. Few of the eggs laid on the native watermilfoils hatched. Our results suggest that when its density is high, E lecontei can have impacts on some native watermilfoil species. However, due to reduced fecundity on native watermilfoils, E lecontei will probably have little impact on the native species. E. lecontei appears to be an example of a native biological control agent that can reduce the abundance of an exotic species without a significant negative impact on closely related, non-target species. (C) 2003 Elsevier Science B.V. All rights reserved.